Revival Mechanism and Prevention Measures of Composite Landslides: A Case Study of the Wenma Expressway Composite Landslide

Round 1

Reviewer 1 Report

Dear authors,

Kindly find the attachment of the detailed comments.

Thank You,

Comments for author File: Comments.pdf

Author Response

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Reviewer 2 Report

Some bibliographic citations do not appear in the text.

Figure 1 does not specify the meaning of some symbols shown in the drawing.

Figure 3 shows formations that are not specified in the legend of figure 2.

For a better understanding of the landslide phenomenon, Figure 4 could be redrawn on a topographic basis.

Figure 7 is difficult to read.

Author Response

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Reviewer 3 Report

The Authors investigated the behaviour of a composite landslide that interferes with a relevant infrastructure. Structurally complex formations outcrop in the area and the presence of a fault added severe stress and strain to the soil, making the landslide hard to be analysed. The case study is therefore potentially interesting both for academics and practitioners, but require a lot of efforts to collect, analyse and present the information so as to obtain a consistent picture of the problem. Unfortunately, the current version of the paper contains serious flaw that prevent it to be published.

In the following, I try to list the most problematic aspects of the paper and I recommend the Authors to carefully address all the points before any resubmission of the manuscript.

 

1.       the GEOLOGIC/GEOTECHNICAL CONDITION of the landslide area is not clearly outlined. The presence of structured, weak and altered formations, flysch/shale and faulted mudstone have to be properly addressed from a geotechnical perspective.

See, among others:

A-      Marinos, V. 2019, "A revised, geotechnical classification GSI system for tectonically disturbed heterogeneous rock masses, such as flysch", Bulletin of Engineering Geology and the Environment, vol. 78, no. 2, pp. 899-912.

B-      Cotecchia, F., Vitone, C., Santaloia, F., Pedone, G. & Bottiglieri, O. 2015, "Slope instability processes in intensely fissured clays: case histories in the Southern Apennines", Landslides, vol. 12, no. 5, pp. 877-893.

C-      Ruggeri, P., Fruzzetti, V.M.E. & Scarpelli, G. 2020, "Design strategies to mitigate slope instabilities in structurally complex formations", Geosciences (Switzerland), vol. 10, no. 2

D-      Bromhead, E.N. Reflections on the residual strength of clay soils, with special reference to bedding-controlled landslides. Q. J. Eng. Geol. Hydrogeol. 2013, 46, 132–156

Note that “strike” is the orientation of the line between the horizontal plane and the fault plane and cannot take values exceeding 180°. It is suggested to describe the discontinuities orientations by using dip direction and dip.

It is very strange that the failure surface cut vertical oriented bedding planes. Check carefully the possibility that other discontinuity families are present or that the joints of unit D1b are orientated parallel to the ground surface up to the depth of the failure surface and not only at shallower depth.

Please, add the inclinometer profiles on the geologic cross-section, to demonstrate the position of the failure surface.

Fault 1 should be represented in the Figure 1 indicating the dip direction and dip angle.

2.       INCLINOMETER READINGS. Why don’t you use the term “inclinometer” in place of “deep deformation monitoring of borehole”?

It is not clear if the inclinometer reading is made by removable probe or if there are in-place probe. In both case it is weird the data markers do not correspond to the change of direction of the deformation line. How do you explain that?

Moreover, it should be useful to draw the displacement recorded at some depth against time, to have a visual representation of the velocity trend with time.

Please, limit the digits to the significative ones and use “mm/day” and not “mm/d”.

Please note that two inclinometer readings are not enough to demonstrate the different velocity of three portion of the landslide body.

3.       GROUNDWATER. This aspect is poorly analysed and badly explained in the paper. No piezometric data are included and information appears inconsistent (compare sentences at L233-236). You stated that more than an aquifer is hosted in the landslide, you observed three springs in the slope but the modelling includes only a general phreatic level under the ground surface. The presence of open cracks suggests a strong driving force exerted by the water filling the cracks during rainfall. This aspect is clearly different from the reduction of effective stress level on the slipe surface related to the groundwater level.

4.       MECHANISM OF LANDSLIDE REACTIVATION. The saturation of the landslide body has the primary effect to reduce the effective stress and consequently the shear strength of soil, according to the Terzaghi principle. I wonder why the Authors focus their attention on the soil weight increment, that is a secondary aspect of limited relevance especially for clay soils that are generally always close to saturation due to suction effect. Given this I do not understand the meaning to test on the shear test apparatus natural not saturated samples. The suction effect without pore pressure measurements during testing alter the shear strength in an unquantified way. So, those results are meaningless in my opinion.

5.       SECTION 5.1 is usefulness. It is necessary to explain the characteristic of the software you are using and the boundary condition so that the calculation is reproducible. At the current level, the analysis with SEEP/w is not verifiable. Note that due to the complexity of the groundwater situation, a so simplified analysis appears of poor usefulness.

6.       STABILITY ANALYSIS. How did you obtain the results shown in Table 3 and Figure 18? No details about SLOPE/w analyses have been given. Note that a factor of safety moving from 1.00 to 1.16 with 30 m of groundwater depression is absolutely not credible.

 

Other aspects:

7.       Figure 6: it is not clear the relevance of the statistical analysis of crack length of Figure 6. What is the idea of the Authors? Why the widening of the cracks is not reported? I think the presence of the cracks means the “landslide engine” is the foot of the slope and the movement is retrogressive. Is a cut of the slope foot that originated the landslide or it is an old landslide?

 

Author Response

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Round 2

Reviewer 3 Report

Dear Authors, you can find my opinion in the attached file.

Comments for author File: Comments.pdf

Author Response

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Round 3

Reviewer 3 Report

Dear Authors, this is my third round of revision on the paper. I carefully read your replies to my observations and verified the revised paper. I confirm my general idea that the paper may be of general interest, in the line of researches related to impact of infrastructures on the territory, especially when there are complex soils and I acknowledge to the Authors an improvement of the current version.

However, I believe that the substantial variation of the slope Safety Factors presented in the current version of the manuscript, very different from the previous ones, raise concerns about the reliability and robustness of the numerical stability analyses carried out by the Authors. I have asked for details and clarifications, but the provided data still impede a complete understanding of the presented analysis.

So, my final conclusion is that the first part of the paper, in which the case study is presented and discussed, is worth to be disseminated even though the second part is still weak. For this reason, I think the paper can only be accepted if:

-          it is clearly specified in the Introduction, in the Section 5 and in the Conclusion that the presented analyses are in a preliminary stage (so, e.g., the title of section 5 should be: Preliminary stability analysis of the landslide);

-          in Table 1 and Table 2, the cohesion and friction angle should be rounded to the nearest integer (two decimal places are meaningless from a geotechnical perspective);

-          to support, with the addition of some citations of reference papers, the relevance of present shear strength parameters measured on natural samples (without saturation) - I still think it is meaningless from a geotechnical perspective.

Author Response

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